Can a licensed mental health therapist who is an employee deduct training for extra certifications and text books from the training?

No.   W-2 employees cannot deduct job-related expenses on a federal return.   Your state laws might be different.

https://ttlc.intuit.com/questions/4482873-which-federal-tax-deductions-have-been-suspended-by-tax-reform

Even though this training was for an additional certification in my field?

As xmasbaby0 indicated, a W-2 employee can not deduct expenses related to their job.  This also more commonly applies to business miles.  I suggest that you look into or encourage an employee reimbursement program through your employer